Autofocus system for CMOS imaging sensors

Abstract

The present invention includes an imaging apparatus that includes a two dimensional array of pixel sensors. Each pixel sensor includes a main photodiode, an autofocus photodiode, and a microlens that concentrates light onto the main photodiode and the autofocus photodiode. The imaging array of pixel sensors includes first and second autofocus arrays of pixel sensors, the pixel sensors in the first autofocus array of pixel sensors having the autofocus photodiodes positioned such that each autofocus photodiode receives light preferentially from one half of the microlens in that pixel sensor, and the pixel sensors in the second autofocus array of pixel sensors having each autofocus photodiode positioned such that each autofocus photodiode receives light preferentially from the other half of the microlens in that pixel sensor. The autofocus photodiodes can be constructed from the parasitic photodiodes associated with the floating diffusion nodes in each pixel sensor or conventional photodiodes.

What is claimed is: 
     
       1. An apparatus comprising a two dimensional array of pixel sensors, each pixel sensor comprising:
 a main photodiode; 
 an autofocus photodiode; and 
 a microlens that concentrates light onto said main photodiode and said autofocus photodiode, 
 said two-dimensional array of pixel sensors comprising first and second autofocus arrays of pixel sensors, said pixel sensors in said first autofocus array of pixel sensors having said autofocus photodiode positioned such that each autofocus photodiode receives light preferentially from one half of said microlens in that pixel sensor and said pixel sensors in said second autofocus array of pixel sensors having each autofocus photodiode positioned such that each autofocus photodiode receives light preferentially from said other half of said microlens in that pixel sensor. 
 
     
     
       2. The apparatus of  claim 1  wherein said autofocus photodiode comprises a pinned photodiode. 
     
     
       3. The apparatus of  claim 2  wherein said main photodiode is characterized by a main photodiode area and said pinned photodiode is characterized by a pinned photodiode area, said main photodiode area being greater than said pinned photodiode area. 
     
     
       4. The apparatus of  claim 1  wherein said autofocus photodiode comprises a parasitic photodiode associated with a floating diffusion node in each of said pixel sensors. 
     
     
       5. The apparatus of  claim 1  wherein said pixel sensors in said first autofocus array of pixel sensors having autofocus photodiodes positioned such that each autofocus photodiode receives more than 80 percent of said light from one half of said microlens in that pixel sensor and said pixel sensors in said second autofocus array of pixel sensors having autofocus photodiodes positioned such that each autofocus photodiode receives light preferentially from said other half of said microlens in that pixel sensor. 
     
     
       6. The apparatus of  claim 1  wherein said pixel sensors in said first autofocus array of pixel sensors having autofocus photodiodes positioned such that each autofocus photodiode receives more than 90 percent of said light from one half of said microlens in that pixel sensor and said pixel sensors in said second autofocus array of pixel sensors having autofocus photodiodes positioned such that each autofocus photodiode receives light preferentially from said other half of said microlens in that pixel sensor. 
     
     
       7. The apparatus of  claim 1  further comprising
 a camera lens that images a scene to be photographed onto said two dimensional array of pixel sensors; 
 an actuator that moves said camera lens relative to said two-dimensional imaging array in response to an autofocus signal from a controller, said controller being configured 
 to cause said pixel sensors to be exposed to light from a scene to be photographed for an autofocus period of time; 
 to obtain signals from each of said pixel sensors in said first and second autofocus arrays of pixel sensors indicative of an amount of light received during said autofocus period of time; and 
 to generate said autofocus signal such that a predetermined portion of said scene will be in focus on a predetermined region of said two dimensional array of pixel sensors. 
 
     
     
       8. The apparatus of  claim 7  wherein generating said autofocus signal comprises computing a cross-correlation function of signals from said autofocus photodiodes in said first autofocus array of pixel sensors with a signal from said autofocus photodiodes in said second autofocus arrays of pixel sensors. 
     
     
       9. The apparatus of  claim 7  wherein said main photodiodes of said pixel sensors in said two-dimensional array of pixel sensors are organized as a uniform array with equal spacing in each dimension of said two dimension array, and wherein said autofocus photodiodes form a non-uniform array. 
     
     
       10. The apparatus of  claim 9  wherein said first autofocus arrays of pixel sensors is a mirror image of said second autofocus array of pixel sensors. 
     
     
       11. The apparatus of  claim 7  wherein said controller generates a first image of said scene using said main photodiodes in said two-dimensional array of pixel sensors including said first and second autofocus arrays of pixel sensors. 
     
     
       12. The apparatus of  claim 1  wherein said first autofocus array of pixel sensors comprises a first linear array of said pixel sensors and said second autofocus array of pixel sensors comprises a second linear array of said pixel sensors configured as a mirror image of said first linear array of pixel sensors. 
     
     
       13. The apparatus of  claim 1  wherein said pixel sensors comprise a plurality of color filters of different colors, one of said plurality of color filters being disposed under said microlens in each of said two-dimensional array of pixel sensors, said first autofocus array being characterized by a first number of color filters of each color contained in said first autofocus array of pixel sensors and said second autofocus array being characterized by a second number of color filters of each color contained in said second autofocus array of pixel sensors, said first and second numbers being substantially equal. 
     
     
       14. The apparatus of  claim 11  wherein said controller outputs a light intensity measurement determined from said autofocus photodiodes in each of said pixel sensors together with said first image.